The invention is directed to a device and method for optoelectronic distance measurement, wherein an intensity-modulated main light beam emitted by a main light emitter is directed to a measurement object at a distance, wherein the distance (D0) of a measurement object from an observation point is to be measured, and a light scattered at the observation point reaches a main photoreceiver via reception optics, and a branched part of the main light beam is simultaneously directed to a reference photoreceiver via a first known reference distance (D1), an intensity-modulated reference light beam emitted by a reference light emitter reaches the reference photoreceiver via a second known reference distance (D2), and a part of the reference light beam reaches the main photoreceiver via a third reference distance (D3), and wherein the signals delivered by the main photoreceiver and the reference photoreceiver are subjected to a comparative signal evaluation for obtaining a corrected measurement signal, wherein the light intensities of the main emitter and the reference emitter are simultaneously intensity-modulated at different frequencies, wherein the signal mixtures supplied by the main receiver and the reference receiver, each of the signal mixtures contains a signal component with the intensity modulation frequency of the main emitter and a signal component with the intensity modulation frequency of the reference receiver, are converted to an intermediate frequency range containing two frequency components, wherein one frequency component is formed by the signal of the reference emitter and the other frequency component is formed by the signal of the main emitter, and the separation of the phase information contained in the two simultaneously occurring intermediate frequency signals is based on the different frequencies in the intermediate frequency range and the different modulation frequencies for the intensity modulation of the main light beam and reference light beam for purposes of comparative signal evaluation.
Optoelectronic measurements of distances up to 100 m with accuracy within a few millimeters have gained importance for numerous applications, particularly in the construction and the plant engineering industries. Such distance measurement systems should be as dynamic as possible making it possible to process very weak signals as well as strong signals. Such measurement systems render superfluous the use of defined target marks on the object whose distance from an observation point is to be determined. The possibility of direct distance measurement at determined surfaces, i.e., without the use of target marks, makes possible reduced manufacturing times and cost savings accompanied by lower manufacturing tolerances, particularly in the industries mentioned above.
Processes and devices for accurate optoelectronic distance measurement are known. In most cases, as well as in the case of the invention, a preferably sine-shaped intensity-modulated beam from a light source, particularly a laser diode, is directed onto an object to be measured. The intensity-modulated light, which is backscattered by this measurement object, is detected by a photodiode. The distance to be measured is given by the phase shift of the sinusoidal modulated light intensity backscattered from the measurement object in relation to the emitted light intensity from the light source.
A principal difficulty in high-precision distance and phase measurement systems of the type mentioned above is the elimination of temperature-dependent and aging-dependent parasitic phase changes in the light source, that is, particularly in the laser diode transmitter and in the photodiode receiver. Various methods are known for countering this difficulty.
One possibility, described in EP 0 701 702 B1, is to use a mechanically switchable reference distance. In this case, an intensity-modulated laser beam is initially directed, in a first measurement, to the measurement object and then, in a second reference distance measurement, is guided directly to the photoreceiver via a tiltable mirror. The influences of temperature and aging on the structural component parts are eliminated by subtracting the measured phases. However, since widely varying optical reception power must be expected with alternating distance and reference distance measurement, a measurement error arising in this way is not eliminated. Another substantial disadvantage of this concept is the use of moving mechanical components, which limit the reliability and service life of the entire measurement system.
Other known distance measurement devices of the type under discussion, which are described in DE 196 43 287 A1, work with a reference photoreceiver and a main photoreceiver. In this case, a part of the intensity-modulated laser light is directed to the measurement object and then to the main photoreceiver and another part which is divided from the laser light beam is guided directly to the reference photoreceiver. No moving mechanical switch is required since the reference photoreceiver is constantly illuminated during measurement. However, in this concept, while the phase response of the laser diode transmitter is eliminated, the phase behavior of the reception components which change over time and are generally different for the measurement arm and reference measurement arm are not eliminated. Further, in distance measurement devices of this type, sharply different reception power in the two arms resulting in further phase errors must also be taken into account.
In another known optoelectronic distance measurement device (see U.S. Pat. No. 4,403,857), two laser emitters and two photodiode receivers are used to eliminate the above-mentioned phase errors. In this device, a portion of the intensity-modulated output of a main light emitter is guided directly onto the measurement object, from which it arrives at the main photoreceiver as scattered light. Another portion of this transmitted output is guided via an exact known first reference distance to a reference photoreceiver. Further, there is a reference light emitter whose delivered output is likewise intensity-modulated and a portion of which reaches the main photoreceiver via a second reference distance, while another portion is guided directly to the reference photoreceiver via a third reference distance.
The main light emitter and the reference light emitter are activated successively via an electronic changeover switch. This measurement principle requires no mechanical changeover switch. In addition, phase changes caused by temperature and aging are completely eliminated in the transmitting unit as well as in the reception unit. However, since substantial differences in reception power must be taken into account in measurements with the signals of the main light emitter and reference light emitter, the phase errors resulting from this are also not eliminated in the concept upon which this known distance measurement device is based. Phase errors depending on reception power are particularly noticeable with avalanche photodiodes (APD) which are preferred as main receivers because of other advantages. At high amplifications, saturation effects gradually come about as output increases, so that avalanche gain is dependent on the received output. Therefore, there occurs, in addition, an output-dependent phase rotation in the case of reception of high-frequency-modulated optic radiation. Further, the generated charge in the barrier layer of the APD varies with the reception power, so that barrier layer distance as well as barrier layer capacity are influenced. As the barrier layer capacity changes, so does the phase behavior of the low-pass formed by it. With high APD gain factors, a phase rotation of greater than 5xc2x0, as a rule, can be brought about in this way with a reception power variation of two orders of magnitude.
It is thus the object of the invention to provide a distance measurement method and a device operating by this method by which a highly accurate distance measurement can be achieved and which is completely independent from phase errors depending on temperature, aging and reception power. Mechanical or electronic changeover switches are dispensed with and the total measuring time for obtaining reliable measurement results is appreciably reduced.
In a method for optoelectronic distance measurement according to the invention, the invention is characterized in that the light intensities of the main emitter and reference emitter are modulated simultaneously at different frequencies, wherein the signal mixtures supplied by the main receiver and reference receiver, each of which signal mixtures contains a signal component with the intensity modulation frequency of the main emitter as well as a signal component with the intensity modulation frequency of the reference emitter, are converted to an intermediate frequency range containing two frequency components, wherein one frequency component is formed by the signal of the reference emitter and the other frequency component is formed by the signal of the main emitter, and in that the separation of the phase information contained in the two simultaneously occurring intermediate frequency signals is carried out based on the different frequencies in the intermediate frequency range and the different modulation frequency for the intensity modulation of the main beam and reference beam for purposes of comparative signal evaluation.
Advantageous further developments of this distance measurement method according to the invention are defined in the dependent patent claims.
A device according to the invention for optoelectronic distance measurement with the features of the invention is characterized, according to the invention, by a device by which the light beams emitted by the main emitter and by the reference emitter can be intensity-modulated simultaneously with different frequencies in each instance.
Advantageous constructions of this distance measurement device according to the invention are likewise defined in further dependent patent claims.
In a manner similar to the distance measurement process described in U.S. Pat. No. 4,403,857, two light transmitters, particularly lasers, and two photodiode receivers are used in the present invention. However, according to the invention, in contrast to this known method, the light of the first light transmitter, designated as main emitter, which is preferably sinusoidal intensity-modulated by a first modulation frequency f1, is directed to the surface of a measurement object. The light which is backscattered from the measurement object and which is likewise intensity-modulated reaches the second photoreceiver, designated as main receiver, for example, via reception optics. At the same time, a portion of the modulated light of the main emitter is guided directly via a first reference distance to the second photoreceiver, designated as reference receiver. The reference emitter is intensity-modulated, likewise preferably in sinusoidal manner, with a second modulation frequency. A portion of its modulated optical beam reaches the main receiver via a second, known reference distance and particularly via a scattering medium, while another component of its modulated optical beam arrives at the reference receiver via a third reference distance.
The two receivers are simultaneously acted upon by the two emitter signals, so that, in contrast to the distance measurement method described in U.S. Pat. No. 4,403,857, no changeover switch is required and the measurement time is appreciably reduced. The photoreceivers convert the detected modulated optical outputs into photocurrents, which are subsequently converted into voltages, preferably, by transimpedance amplifiers.
The two signal voltages obtained in this manner are subsequently converted by associated mixers into suitable intermediate frequency ranges using a locally generated frequency and are then evaluated after analog-to-digital conversion of a signal evaluation for error-free determination of the phase shift caused by the signal rise time or transit time and accordingly for determining the distance.